Ammonoids and nautiloids from the earliest Spathian Paris Biota and other early Spathian localities in southeastern Idaho, USA

Abstract

Intensive sampling of three earliest Spathian sites represented by the Lower Shale unit and coeval beds within the Bear Lake vicinity and neighboring areas, southeastern Idaho, yielded several new ammonoid and nautiloid assemblages. These new occurrences overall indicate that the lower boundary of the Tirolites beds, classically used as a regional marker for the base of the early Spathian, and therefore the regional Smithian/Spathian boundary, must be shifted downward into the Lower Shale unit and coeval beds. Regarding ammonoids, one new genus (Caribouceras) and two new species (Caribouceras slugense and Albanites americanus) are described. In addition, the regional temporal distribution of Bajarunia, Tirolites, Columbites, and Coscaites is refined, based on a fourth sampled site containing a newly reported occurrence of the early Spathian Columbites fauna in coeval beds of the Middle Shale unit. As a complement to ammonoids, changes observed in nautiloid dominance are also shown to facilitate correlation with high-latitude basins such as Siberia during this short time interval, and they also highlight the major successive environmental fluctuations that took place during the late Smithian–early Spathian transition.

Introduction

Pioneering contributions regarding our knowledge of Early Triassic ammonoids from the northern part of the western USA basin were accomplished by White (1879), Hyatt and Smith (1905) and Smith (1932). They were the first to report and illustrate various taxa from the Smithian–Spathian interval in the Bear Lake area of southeastern Idaho (Guex et al., 2010, Jenks et al., 2015; Fig. 1). Smith (1932) also proposed the first regional Early Triassic biozonation based on these organisms by using their successive occurrences observed in several localities within this area. In particular, he recognized the iconic middle and late Smithian Meekoceras and Anasibirites beds, and the early Spathian Tirolites and Columbites beds. Kummel, 1943, Kummel, 1954, Kummel, 1969 continued the exploration of this area and completed the biozonation for the middle Spathian. Kummel, 1954, Kummel, 1957 also documented the major Lower Triassic lithologic units found in the Bear Lake area, provided ammonoid zonal-based ages for each unit and suggested correlation of these units between studied sections (Fig. 2). Additionally, Kummel (1953) reviewed the rare Early Triassic coiled nautiloid occurrences known for this area and noted that a large majority came from the early Spathian Columbites beds. Recently, Guex et al., 2005a, Guex et al., 2005b, Guex et al., 2010 and Jenks et al. (2013) significantly refined these previous works and provided an ammonoid zonation for the Spathian, with particular emphasis on the early Spathian, achieving a much higher resolution than elsewhere in the world (see Jenks et al., 2015 for a review of global Early Triassic correlations).

Here we focus on ammonoids and nautiloids recently found in the earliest Spathian of the Bear Lake area, which yields specimens of both clades (Brayard et al., 2017). Based on data from the Paris Biota and coeval beds in neighboring sites in the Bear Lake area (Fig. 1), we document the first reported occurrence of the ammonoids Tirolites harti, Tirolites aff. cassianus and Bajarunia cf. pilata, as well as a new genus Caribouceras nov. gen. and two new species Caribouceras slugense nov. gen., nov. sp. and Albanites americanus nov. sp. from the earliest Spathian platy shale and siltstone interval underlying the Middle Limestone unit and coeval beds. The Middle Limestone unit and coeval beds, which contains Tirolites harti, has been commonly considered as marking the beginning of the early Spathian in southeastern Idaho. This unit is overlain by the classic early Spathian assemblage of Columbites parisianus and Tirolites smithi, both species being common in the Bear Lake area. These new ammonoid occurrences in the Lower Shale unit or coeval beds, therefore, necessitate a downward revision of the Smithian/Spathian boundary (SSB) in the region and also modify the assumed correlation among sections in this area.